As young children acquire new ways to coordinate actions and represent events, their joint engagement with others is transformed. These transformations are both dependent on and affect the acquisition of language and social understanding. This application's long-term objective is to document typical and atypical trajectories of joint engagement development, especially those taken by young children with developmental disorders, including autism spectrum disorder (ASD). To serve this objective, this competitive renewal application concentrates on auditory joint engagement-the active sharing of sounds with a social partner-during parent- child interactions. Three specific aims are pursued. First, the typical developmental course of auditory joint engagement will be characterized by observing 60 typically developing (TD) children at 5 time points between 12 and 36 months of age as they interact with their parents. These semi-naturalistic observations will be made using the Communication Play Protocol-Auditory (CPP-A), a modified version of our long-standing CPP observational procedure. The CPP-A generates excellent video records of parent-child communication, including one taken by a wearable headcam that captures the parent's point of view. This study will detail how four types of sounds-child-focused speech, music, animal sounds, and mechanical noises-are responded to, drawn into interactions, and elaborated through gestures and language. Second, the effects of ASD and other developmental disorders (DD) on auditory joint engagement will be assessed by observing 180 children divided into three groups (ASD, DD and TD) in the CPP-A at the time of early diagnosis (18-24 months) and at 36 months. Of particular interest will be how the type of sound affects auditory joint engagement, especially in light of reports that ASD differentially affects apprehension of speech and social sounds. Third, variations in children's auditory joint engagement will be related to the development of language and social understanding to discern how it both reflects and fuels advances in these crucial accomplishments. Fulfilling these aims will provide an unprecedented view of how children react to and share the stream of speech, music, and environmental sounds that surround them and at times are the focus of their interaction with parents. Our findings will both clarify central aspects of communication development and contribute to refining early interventions that seek to ameliorate the pervasive effects of joint attention deficits and aberran auditory reactions in children with developmental disorders.